The invention relates to a volute shaped pump casing according to the preamble of claim 1, to a method of manufacturing a volute shaped pump casing according to the preamble of claim 9 and to a centrifugal pump including such a volute shaped pump casing.
Volute casing pumps are very common. Their characteristic feature is the volute-shaped pump casing which, as a rule, makes this pump type recognizable from the outside. A cross section through a typical volute-shaped pump casing is shown in FIG. 1A. Volute casing pumps are usually single stage. Two stage and multistage volute casing pumps are used less frequently. In some applications a volute shaped casing is provided only for the last stage. Single suction and double suction volute casing pumps are both used frequently.
A volute shaped casing generally includes a chamber designed to house at least one impeller being usually of the radial or mixed flow type and mounted on a shaft for rotation when driven by a motor. The casing further includes a volute shaped chamber section to collect pumped medium and a discharge channel section to guide the medium out. The discharge channel section can be arranged tangentially to the volute casing, or arranged radially by providing a swan neck. A suction channel section is favorably arranged axially in case of bearings arranged only at one side of the impeller, and radially or tangentially in case of bearings at either side of the impeller.
In its simplest embodiment of a single volute, the casing can be broadly sub-divided into two main sections consisting of a downstream chamber section and the upstream discharge channel section. The plane or section at which the chamber and channel meet is generally defined as the throat. The leading edge of the throat which separates or guides the flow from the chamber into the channel is designated cutwater lip or cut water and for any given length the top and bottom surface extending beyond the lip is termed the tongue. In the case of a casing with a plurality of volutes or flow channels disposed around an impeller the number of lips will usually be equal to the number of volutes or flow channels. In such cases the wall separating two neighboring flow channels such as a volute shaped chamber section and a neighboring discharge channel section is named a rib or splitter rib or splitter rib wall.
On rotation a radial thrust is generated in centrifugal pumps by the interaction of the impeller and the pump casing. In a volute casing pump having a single volute the radial thrust becomes a minimum when the pump is operated at the best efficiency point. When the pump is operated off the best efficiency point the radial thrust increases. Volute casing pumps having two or more volutes disposed around an impeller were developed to reduce the radial thrust generated by operating the pump off the best efficiency point. The splitter ribs provided in the casing of such pumps can influence the stability in the head performance curve and instabilities in the head performance curve can manifest in such pumps. This is thought to be due to flow separation at either side of the splitter rib.
A curved splitter rib with a given thickness can be defined along a mean camber line (see e.g. FIG. 2). The mean camber line is a reference design line equidistantly positioned at all points between the upper and lower surfaces of the splitter rib. For ease of interpretation and design the mean camber line of a curved splitter rib can be unwrapped to a straight line. The result is that the top and bottom surfaces of the splitter rib are always symmetric along this mean camber line regardless of what the upper and lower surface profiles looked like in the wrapped configuration.
In the current state of technology the splitter rib designs incorporate a constant or variable thickness along its length. In the case of splitter ribs with a constant thickness t, the position at which the thickness starts to be constant is at a distance L′ from the leading edge of the splitter rib, with L′ being measured in the direction of the unwrapped mean camber line and being smaller than 1.2 times the constant thickness t (see FIGS. 4A-4C). In case of splitter ribs with variable thickness, on the other hand, the maximum thickness position will be located at a wrap angle higher than 60 degrees starting from the leading edge of the splitter rib. The conventional splitter rib design has, however, not been able to overcome the instability in the head performance curve described above.